Modulation of high power lasers, using either acousto-optic or electro-optic systems, presents efficiency problems. With acousto-optic modulators, there is an inevitable trade off between switching rise time and diffraction efficiency. In other words the fastest data throughput is accompanied with the minimum optical efficiency. Electro-optic modulators, which predominantly rely on polarising optics can achieve optical efficiencies of approaching 50%.
Normal optical recording methods benefit from recording systems which are highly inefficient, with only a small percentage of the generated laser energy being used to expose the final image on film. With silver halide films or xerographic processes, where the required imaging sensitivity is of the order of 15-35 mJ/M.sup.2, optical economies can be achieved by using optical systems which do not collect or image all of the energy generated. Thermal optical recording, however uses media which is energy intensive having sensitivities in excess of 100 mJ/cm.sup.2. consequently more efficient optical systems, both imaging and modulation are required for this media, otherwise severe problems will be encountered both in the provision of a dump for the waste heat, and in economic penalties by requiring a much higher power laser than that just required for imaging the media.
Imagebars comprising a plurality of light gates present an alternative approach to imaging high resolution optical recording systems in pre-press environments. The traditional approach has been to use a segmented section in a sub raster scanning system, using either flat bed or external drum approaches. Previous methods have used electro-optic light gate arrays, based on PLZT, ferro-electric liquid crystal or magneto-optic switching systems.
There are a number of reflective optical systems which can be used for imagebar construction which result in a substantial improvement in optical throughput efficiency. The first, described by Hornbeck in U.S. Pat. No. 4,441,791 and illustrated in application to xerographic printing in U.S. Pat. No. 4,571,063, uses a silicon deformable mirror spatial light modulator. With these types of systems, the incident light is reflected onto is the imaging media or onto an optical stop.
U.S. Pat. No. 4,441,791 discloses a light modulator comprising a light-reflective metallized membrane defining a deformable mirror disposed over a semiconductor substrate of one conductivity type. A matrix of floating metallic field plate members is disposed on an insulating layer covering the substrate to define an array of air gap capacitors for line addressing by the field effect address transistors. The floating metallic field plates are opaque to light and prevent photocharge generation in the active regions of the matrix array of field effect address transistors. The metallized membrane is spaced from the field effect address transistors and the metallic floating field plates by an upstanding semiconductor grid structure which is formed on the insulating layer of the semiconductor substrate and defines gate electrodes for the address transistors. The metallized membrane is mounted on the upstanding semiconductor grip structure by molecular bonding to the contact members disposed over the semiconductor grid structure. The metallized membrane is formed of a polymer of nitrocellulose as a flexible carrier layer on at least one surface of which is disposed a thin metallic coating providing a light reflective surface. Each transistor in the array of field effect address transistors is line-addressable, and the metallized membrane in each cell of the matrix array of air gap capacitors is deflectable inwardly toward the substrate in response to the signal on the address transistor corresponding thereto. Should a potential above a predetermined magnitude be placed on an individual air gap capacitor, the metallized membrane will transfer charge to the floating field plate and return to zero deflection. The floating field plate thereby not only acts as a light-blocking layer, but also prevents voltage-induced collapse of the metallized membrane to the surface of the semiconductor substrate.
Various problems have arisen with deformable mirror spatial light modulators of the type disclosed in U.S. Pat. No. 4,441,791. In particular, problems have occurred due to pixel sticking, fracture or fatigue failure.
Furthermore, the angular tolerances required of the individual modulators in the switched state is very high, for such devices to be used in printing applications. The high angular movement also precludes their use in fast rise time applications. For printing, the rise time of the exposure should be less than one-third of the required exposure time, otherwise adjacent pixels will have non reciprocal exposure times. This requirement inevitably limits the printing bandwidth of these devices.